Add Druid e2e tests for multi node etcd

[seshachalam-yv]

How to categorize this PR?

/area testing
/kind enhancement

What this PR does / why we need it:
This PR adds e2e tests for multi-node etcd

Etcd cluster operations:

• Creation of 3-member etcd cluster
• Hibernation
  • Scale down from 3->0
  • Scale up from 0->3
• Quorum is lost
• Single node is restored
• Zero downtime rolling updates ---> skipped (need to investigate why etcd cluster is not healthy while rolling update)
• Zero downtime maintenance operations - Defragmentation
• Deletion of 3-member etcd cluster

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
This PR made changes on top of #296.
Introduced multi-node e2e tests and helper functions in following files

• test/e2e/multi_node_etcd_test.go
• test/e2e/utils.go

Depends on the following issues/PRs:
#296

Release note:

[gardener-robot]

@seshachalam-yv Thank you for your contribution.

[gardener-robot-ci-3]

Thank you @seshachalam-yv for your contribution. Before I can start building your PR, a member of the organization must set the required label(s) {'reviewed/ok-to-test'}. Once started, you can check the build status in the PR checks section below.

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⚠️ GitGuardian has uncovered 7 secrets following the scan of your pull request.

Please consider investigating the findings and remediating the incidents. Failure to do so may lead to compromising the associated services or software components.

🔎 Detected hardcoded secrets in your pull request

GitGuardian id	Secret	Commit	Filename
-	RSA Private Key	967218c	test/e2e/resources/certs/ca.key	View secret
-	RSA Private Key	967218c	test/e2e/resources/certs/client.key	View secret
-	RSA Private Key	967218c	test/e2e/resources/certs/server.key	View secret
-	RSA Private Key	b95b50e	test/e2e/resources/certs/client.key	View secret
-	RSA Private Key	b95b50e	test/e2e/resources/certs/server.key	View secret
-	Generic Private Key	b95b50e	test/e2e/resources/tls/client.key	View secret
-	Generic Private Key	b95b50e	test/e2e/resources/tls/server.key	View secret

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[timuthy]

Thanks for the PR and effort @seshachalam-yv. Since #296 has been merged, can you please rebase this PR and adapt to the latest structural changes introduced in test/e2e/etcd_backup_test.go?

[timuthy]

/needs rebase

[seshachalam-yv]

Closed this PR, opened another PR to track this #418